Observation on volcanic rocks types and alteration around Dieng geothermal field in Central Java

Abstract

Volcanic rocks around the Dieng geothermal field consists of tuff breccia, basalt, basaltic andesite, pyroxene andesite, xenolith andesite. The rocks have undergone alteration is generally weak until the total. Weak alteration generally occurs in basalt, basaltic andesite, pyroxene andesite, xenolith andesite that characterized by the appearance of chlorite-calcite-epidote as a group minerals formed prophylitic alteration at a relatively high temperature of around 230oC. While the strong alteration to the total occurred in the breccia tuff that characterized by the appearance of kaolinite, montmorillonite, illite, halloysite, and cristoballite, which is included as argillic alteration, and can be formed at temperatures around 100oC. From the Northwest and North to South and Southeast, from Wanayasa, Rogo Jembangan, Jalatunda, Siglagah, Sleri, and towards Pulosari, Sikidang, Merdada and Cebong, the rocks were found change from basalt to the South and Southeast transformed into xenolith andesite (trachyte basalt xenolith, diorite xenolith?) and contain the hornblende, biotite minerals. The Changes of rocks composition is likely due to the occurrence of assimilation (?) because of the volcanic rocks penetrated sedimentary rocks of Tertiary age or other penetrating igneous rocks (?). Based on the relationship Niobium (Nb) / Zircon (Zr) * 100 with Yttrium (Y) indicate that volcanic rocks Dieng area is located in a backarc side-volcanoes.